A wide variety of semiconductor devices, and methods of making semiconductor devices, are known. Some of these devices are designed to emit light, such as visible or near-visible (e.g. ultraviolet or near infrared) light. Examples include electroluminescent devices such as light emitting diodes (LEDs) and laser diodes, wherein an electrical drive current or similar electrical signal is applied to the device so that it emits light. Another example of a semiconductor device designed to emit light is a re-emitting semiconductor construction (RSC).
Unlike an LED, an RSC does not require an electrical drive current from an external electronic circuit in order to emit light. Instead, the RSC generates electron-hole pairs by absorption of light at a first wavelength λ1 in an active region of the RSC. These electrons and holes then recombine in potential wells in the active region to emit light at a second wavelength λ2 different from the first wavelength λ1, and optionally at still other wavelengths λ3, λ4, and so forth depending on the number of potential wells and their design features. The initiating radiation or “pump light” at the first wavelength λ1 is typically provided by a blue, violet, or ultraviolet emitting LED coupled to the RSC. Exemplary RSC devices, methods of their construction, and related devices and methods can be found in, e.g., U.S. Pat. No. 7,402,831 (Miller et al.), U.S. Patent Application Publications US 2007/0284565 (Leatherdale et al.) and US 2007/0290190 (Haase et al.), PCT Publication WO 2009/048704 (Kelley et al.), and pending U.S. Application Ser. No. 61/075,918, “Semiconductor Light Converting Construction”, filed Jun. 26, 2008, all of which are incorporated herein by reference.
When reference is made herein to a light at a particular wavelength, the reader will understand that reference is being made to light having a spectrum whose peak wavelength is at the particular wavelength.
FIG. 1 shows an illustrative device 100 that combines an RSC 108 and an LED 102. The LED has a stack of LED semiconductor layers 104, sometimes referred to as epilayers, on an LED substrate 106. The layers 104 may include p- and n-type junction layers, light emitting layers (typically containing quantum wells), buffer layers, and superstrate layers. The layers 104 may be attached to the LED substrate 106 via an optional bonding layer 116. The LED has an upper surface 112 and a lower surface, and the upper surface is textured to increase extraction of light from the LED compared to the case where the upper surface is flat. Electrodes 118, 120 may be provided on these upper and lower surfaces, as shown. When connected to a suitable power source through these electrodes, the LED emits light at a first wavelength λ1, which may correspond to blue or ultraviolet (UV) light. Some of this LED light enters the RSC 108 and is absorbed there.
The RSC 108 is attached to the upper surface 112 of the LED via a bonding layer 110. The RSC has upper and lower surfaces 122, 124, with pump light from the LED entering through the lower surface 124. The RSC also includes a quantum well structure 114 engineered so that the band gap in portions of the structure is selected so that at least some of the pump light emitted by the LED 102 is absorbed. The charge carriers generated by absorption of the pump light move into other portions of the structure having a smaller band gap, the quantum well layers, where the carriers recombine and generate light at the longer wavelength. This is depicted in FIG. 1 by the re-emitted light at the second wavelength λ2 originating from within the RSC 108 and exiting the RSC to provide output light.
FIG. 2 shows an illustrative semiconductor layer stack 210 comprising an RSC. The stack was grown using molecular beam epitaxy (MBE) on an indium phosphide (InP) wafer. A GaInAs buffer layer was first grown by MBE on the InP substrate to prepare the surface for II-VI growth. The wafer was then moved through an ultra-high vacuum transfer system to another MBE chamber for growth of II-VI epitaxial layers used in the RSC. Details of the as-grown RSC are shown in FIG. 2 and summarized in Table 1. The table lists the thickness, material composition, band gap, and layer description for the different layers associated with the RSC. The RSC included eight CdZnSe quantum wells 230, each having a transition energy of 2.15 eV. Each quantum well 230 was sandwiched between CdMgZnSe absorber layers 232 having a band gap energy of 2.48 eV that could absorb blue light emitted by an LED. The RSC also included various window, buffer, and grading layers.
TABLE 1Refer-Thick-Band Gap/encenessTransitionNo.Material(nm)(eV)Comment230Cd0.48Zn0.52Se3.12.15quantumwell232Cd0.38Mg0.21Zn0.41Se82.48absorber234Cd0.38Mg0.21Zn0.41Se:Cl922.48absorber236Cd0.22Mg0.45Zn0.33Se1002.93window238Cd0.22Mg0.45Zn0.33Se →2502.93-2.48gradingCd0.38Mg0.21Zn0.41Se240Cd0.38Mg0.21Zn0.41Se:Cl462.48absorber242Cd0.38Mg0.21Zn0.41Se →2502.48-2.93gradingCd0.22Mg0.45Zn0.33Se244Cd0.39Zn0.61Se4.42.24II-VI buffer246Ga0.47In0.53As1900.77III-V buffer224InP350,0001.35III-VsubstrateFurther details of this and other RSC devices can be found in PCT Publication WO 2009/048704 (Kelley et al.).
Of particular interest to the present application are light sources that are capable of emitting white light. In some cases, known white light sources are constructed by combining an electroluminescent device such as a blue-emitting LED with first and second RSC-based luminescent elements. The first luminescent element may, for example, include a green-emitting potential well that converts some of the blue light to green light, and transmits the remainder of the blue light. The second luminescent element may include a potential well that converts some of the green and/or blue light it receives from the first luminescent element into red light, and transmits the remainder of the blue and green light. The resulting red, green, and blue light components combine to allow such a device, which is described (among other embodiments) in WO 2008/109296 (Haase), to provide substantially white light output.
Some devices provide white light using a pixelated arrangement or array. That is, multiple individual light-emitting elements, none of which emit white light by themselves, are arranged in close proximity to each other so as to collectively form a pixel. The pixel typically has a characteristic dimension or size below the resolution limit of the observation system, so that light from the different light-emitting elements is effectively combined in the observation system. A common arrangement for such a device is for three individual light-emitting elements—one emitting red (R) light, one emitting green (G) light, one emitting blue (B) light—to form an “RGB” pixel. Reference is again made to WO 2008/109296 (Haase), for disclosure of some such devices.
Also of interest to the present application are light sources that are not only capable of emitting white light, but also of changing or adjusting the apparent color of the output. For example, in some cases a “cool” white color may be desired, while in others a “warm” white color may be desired. A given “shade” of white may be plotted as an (x,y) color coordinate on a conventional CIE chromaticity diagram, and can be characterized by a color temperature as is known by those skilled in the art.
U.S. Pat. No. 7,387,405 (Ducharme et al.), for example, discusses lighting systems that provide adjustable color temperature. One such lighting system uses multiple light sources constructed by combining a blue-emitting LED with a layer of yellow phosphor. Some of the blue light is absorbed by the phosphor and re-emitted as yellow light, and some of the blue light passes through the phosphor layer. The transmitted blue light combines with the re-emitted yellow light to produce an output beam having an overall output spectrum that is perceived as nominally white light. Device-to-device variations in phosphor layer characteristics and/or other design details give rise to device-to-device differences in the output spectrum and corresponding differences in perceived color, with some LED/phosphor devices providing a “cool” white color and others providing a “warm” white color. The '405 patent reports that some commercial LED/phosphor devices exhibit color temperatures of 20,000 degrees Kelvin (20,000K) while others exhibit color temperatures of 5750K. The '405 patent also reports that a single one of these LED/phosphor devices allows for no control of color temperature, and that a system with a desired range of color temperature cannot be generated with one device alone. The '405 patent goes on to describe an embodiment in which two such LED/phosphor devices are combined with an optical long-pass filter (a transparent piece of glass or plastic tinted so as to enable only longer wavelength light to pass through) that shifts the color temperature of the devices, and then a specific third LED (an Agilent HLMP-EL 18 amber LED) is added to these filtered LED/phosphor devices to provide a 3-LED embodiment with adjustable color temperature.